This invention relates to a casting core for the water jacket of a cylinder block in a multicylinder, reciprocating-piston internal combustion engine, whose cylinders are arranged in line and are cast in one piece linked by webs, the two halves of the core arranged on the long sides of the cylinder block being held together by the core sections on the front ends of the cylinder block and being secured by supports piercing the cylinder webs. Contrary to this proposal the cores for the water jackets of the conventional types of cylinder blocks, with separated cylinders and passages for the cooling water between these cylinders, are provided with integrated cores for the cooling passages. In these instances no additional supports for securing the two halves of the core are required.